The present invention relates to electroluminescent laminates that include a thin film electroluminescent oxide phosphor layer.
Thin film electroluminescent (TFEL) devices typically include a laminate or laminar stack of thin films deposited on an insulating substrate. The thin films include a transparent electrode layer and an electroluminescent (EL) phosphor structure, comprising an EL phosphor material sandwiched between a pair of insulating layers. A second electrode layer completes the laminate structure. In matrixed addressed TFEL panels the front and rear electrodes form orthogonal arrays of rows and columns to which voltages are applied by electronic drivers, and light is emitted by the EL phosphor in the overlap area between the rows and columns when sufficient voltage is applied in excess of a voltage threshold.
In designing an EL device, a number of different requirements have to be satisfied by the laminate layers and the interfaces between these layers. To enhance electroluminescent performance, the dielectric constants of the insulator layers should be high. To work reliably however, self-healing operation is desired, in which electric breakdown is limited to a small localized area of the EL device: The electrode material covering the dielectric layer fails at the local area, preventing further breakdown. Only certain dielectric and electrode combinations have this self-healing characteristic. At the interface between the phosphor and insulator layers, compatibility between materials is required to promote charge injection and charge trapping, and to prevent the interdiffusion of atomic species under the influence of the high electric fields during operation.
Standard EL thin film insulators, such as SiO2, Si3N4, Al2O3, SiOxNy, SiAlOxNy and Ta2O5 typically have relative dielectric constants (K) in the range of 3 to 60 which we shall refer to as low K dielectrics. These dielectrics do not exhibit the properties required to work well in layers adjacent to oxide phosphors, which have high threshold electric fields. A second class of dielectrics, called high K dielectrics, hold more potential. This class includes materials such as SrTiO3, BaTiO3, PbTiO3 which have relative dielectric constants in the range of 100 to 10,000, and are crystalline with the perovskite structure. While all of these dielectrics exhibit a sufficiently high figure of merit (defined as the product of the breakdown electric field and the relative dielectric constant) to function in the presence of high electric fields, not all of these materials offer sufficient chemical stability and compatibility in the presence of high processing temperatures and/or high electric fields. SrTiO3, BaTiO3, exhibit the required properties to provide good performance in an EL device, when positioned adjacent to oxide phosphors.
In view of the multiple and often conflicting requirements placed on the insulating layers and their interfaces, multicomponent insulator structures have been proposed. Also, it is known in the art that SrTiO3, BaTiO3 can be used in EL devices. For example, U.S. Pat. No. 4,857,802 to Fuyama discusses the use of SrTiO3 and BaTiO3 insulating layers. However, this teaches how to grow the pervoskite structure dielectrics in a [111] orientation to improve its breakdown strength, and only discusses application with sulfide phosphors. The compatibility issues with oxide phosphors, and the incorporation of self-healing breakdown functionality is not addressed.
U.S. Pat. No. 4,547,703 to Fujita teaches the use of a multi-layer insulator comprised of non-self healing dielectric layers combined with self healing dielectric layers. In this case, a self-healing, low K dielectric is adjacent to the sulfide phosphor, and the primary rationale for including the non-self-healing dielectric in the EL device was to increase the capacitance of the insulating layer, thereby increasing the electric field in the phosphor and increasing the charge transfer into the phosphor during emission. The rationale did not include providing electrical and chemical compatibility with the phosphor.
U.S. Pat. No. 4,897,319 to Sun teaches the use of a multi-layered insulator in an EL device. However, in Sun""s devices, no high K dielectrics are employed, and he teaches that it is essential to have a SiON layer (a low K dielectric) adjacent to the sulfide phosphor.
Thus, two component insulators have been proposed in which a low dielectric constant material maintains the charge trapping and injection at the interface with the phosphor, and a high dielectric constant material layer increases the electric field in the phosphor. A high dielectric constant layer increases the field in the phosphor, and a low dielectric constant layer interfaces with an electrode to promote self-healing electrical breakdown.
The teachings of the prior art on TFEL structures are based on the use of doped zinc sulfide as the EL phosphor layer. It would be very advantageous to provide a TFEL device that uses electroluminescent oxides instead of sulphides since the former are less sensitive to atmospheric water vapor and oxygen and so minimal sealing is required in manufacturing the display. Since the interface characteristics between the insulator layer and the phosphor are important in designing a successful EL structure, prior art is not particularly helpful in developing a TFEL stack which uses unrelated material formulations as the EL phosphor layer. While SrTiO3 and BaTiO3 exhibit desirable interface and charge injection properties with oxide phosphors, they also exhibit propagating breakdown mode in thin films.
Therefore, it would be very advantageous to provide thin film electroluminescent structures which use oxide based electroluminescent phosphors and which provide a self-healing breakdown mode of operation. A more electrically robust dielectric layer with a high figure of merit is required adjacent to the phosphor to provide proper electron trapping and charge injection in the presence of high electric fields. At the same time, the material must not react with the phosphor during high temperature processes in manufacture, nor allow chemical reaction or inter-diffusion of chemical species between the phosphor or the adjacent layer in the presence of these high electric fields. Because both bulk and surface properties are important, this is known as the dielectric interface layer.
It is an object of the present invention to develop thin film EL device structures that include the oxide phosphors.
To achieve this objective, thin film SrTiO3 and BaTiO3 have been employed next to the oxide phosphor layer (on one or both sides of the oxide layer), primarily as stable charge injection, and trapping interface layers, and to increase the electric field in the phosphor. Electric breakdown protection through self-healing has been provided by traditional low K dielectrics in combination with an appropriate choice of adjacent electrode. The high dielectric constant materials employed also provide for a high capacitance layer, thereby increasing the electric field in the phosphor and increasing the charge transfer into the phosphor during emission.
In one aspect of the invention there is provided an electroluminescent laminate, comprising;
an electrically insulating substrate;
a conducting metal oxide layer on a surface of the substrate;
an electroluminescent oxide phosphor layer on the conducting layer;
a first dielectric interface layer on the oxide phosphor layer;
a first dielectric layer on the first dielectric interface layer, the first dielectric interface layer having a dielectric constant higher than a dielectric constant of the first dielectric layer; and
a second conducting layer on the first dielectric layer, and wherein at least one of the conducting layer and the conducting metal oxide layer is substantially transparent, and wherein when only the conducting metal oxide layer is substantially transparent the substrate is also transparent. In this aspect of the invention the first dielectric layer and the second conducting layer are characterized by self-healing properties.
In another aspect of the invention there is provided an electroluminescent laminate, comprising;
an electrically insulating substrate;
a first conducting layer on a surface of the substrate;
a first dielectric interface layer on the first conducting layer;
an oxide phosphor layer on the first dielectric interface layer;
a second dielectric interface layer on the oxide phosphor layer;
a first dielectric layer on the second dielectric interface layer;
the first and second dielectric interface layers each having a dielectric constant greater than a dielectric constant of the first dielectric layer; and
a second conducting layer on the first dielectric layer, and wherein at least one of the two conducting layers is substantially transparent, and wherein when only said first conducting layer is substantially transparent said substrate is also transparent.
In this aspect of the invention the first dielectric layer and the second conducting layer are characterized by self-healing properties.